Surface acoustic wave elements are widely used as bandpass filters in communication devices, such as mobile phones. With the higher performance of mobile phones and the like, filters using the surface acoustic wave elements are also required to have higher performance.
The surface acoustic wave elements, however, have a problem of shifting the passband because of changes in temperature. In particular, for example, lithium niobate and lithium tantalate, which are currently used in many cases, have a large electromechanical coupling factor, so that this is very advantageous to achieve broadband filter characteristics. However, both the lithium niobate and lithium tantalate have poor temperature stability.
For example, the temperature coefficient of the change in frequency of the surface acoustic wave filter using lithium tantalate is −35 ppm/° C., which means that variations in frequency in the assumed operating temperature range are large. Thus, it is necessary to reduce the temperature coefficient of the change in frequency.
To fabricate an acoustic wave element having excellent frequency temperature characteristics, it is known to utilize a composite substrate that is produced by bonding a functional substrate made of a piezoelectric material or the like to a supporting substrate made of material exhibiting low thermal expansion (as disclosed in Patent Document 1).
On the other hand, in fabricating a light-emitting element using a nitride-based compound semiconductor, such as GaN, a method is proposed in order to reduce the amount of use of a substrate made of expensive GaN-based semiconductor. The method is known that involves bonding a GaN-based semiconductor substrate to a supporting substrate made of a different material (e.g., ZnO, β-Ga2O3), and then separating most of the semiconductor substrate from the supporting substrate while leaving only a thin layer of the GaN-based semiconductor on the supporting substrate (see Patent Document 3).
In antenna selector switches for use in mobile phones, a silicon on sapphire (SOS) technique has recently attracted attention, in terms of the multimode and multiband as well as reduction in size and cost. This technique involves epitaxially growing Si on a sapphire substrate and forming a CMOS switch by using the Si layer. A feature of the technique is that the use of the insulating sapphire suppresses current leakage to the substrate, and thus is very effective in reducing power consumption. Such a technique is similar to the Si platform technology, enabling integration of a decoder circuit, etc., on one chip, and hence is very advantageous in terms of downsizing and cost reduction. On the other hand, the technique described above sometimes results in degradation of the quality of the Si layer because of the use of a single-crystal sapphire substrate having a different lattice constant from that of Si to epitaxially grow Si on the sapphire substrate. To suppress this phenomenon, a substrate including a single-crystal Si (functional substrate) bonded to a sapphire substrate (supporting substrate) has been studied (as disclosed in Patent Document 4).
The functional substrate and the supporting substrate are bonded together directly or via a bonding layer or an adhesive layer. In either case, to maximize the bonding strength, it is necessary to planarize the surface of the substrate by the chemical-mechanical planarization (CMP) or the like.